1. Field of the Invention
The present invention relates to a start control device for a fuel cell system and, in particular, relates to a technology to heat control valves disposed in a passage of a reaction gas for the fuel cell.
2. Background Art
A fuel cell in which a membrane electrode assembly is provided by sandwiching a solid polymer electrolyte membrane with an anode electrode and a cathode electrode and by sandwiching the outside of the membrane electrode assembly with a pair of separators is known. In this type of fuel cell, when a fuel gas (for example, hydrogen gas) is supplied to a power generating surface of the anode electrode and an oxidant gas (for example, air containing oxygen) is supplied to a power generating surface of the cathode electrode, a chemical reaction takes place, an external circuit collects electrons generated by the electrochemical reaction, and a direct current to be used as electrical energy is output from the circuit as the output energy of the fuel cell. Since oxidant gas (for example, air containing oxygen) is supplied to the cathode electrode, the reactions between hydrogen ions, electrons, and oxygen produce water. Accordingly, fuel cells have attracted attention as driving sources for vehicles because fuel cells have very little effect on the environment.
In general, the operating temperature of the aforementioned type of fuel cell is in a range of 70 to 80xc2x0 C. Since power generation efficiency is low at temperatures below the normal operating temperature, a problem arises in that starting of the fuel cell at a low temperature is quite difficult. Thus, when the fuel cell is used as a power source of a vehicle, a problem occurs in that it takes a long time for the vehicle to start to move when the outside temperature is low, for example, when the outside temperature is below freezing.
For example, a technique to start a fuel cell is proposed in Published Japanese Translation of PCT Application No. 2000-512068, in which the starting of the fuel cell at a low temperature is facilitated by heating the fuel cell accompanied with an accelerated reaction by applying electric power to an external load of the fuel cell.
U.S. Pat. No. 6,103,410 discloses a technique to facilitate starting of a fuel cell at a low temperature by generating heat in the fuel cell due to a combustion reaction initiated by the cathode catalyst when a part of the fuel, that, is, hydrogen is mixed with air and combusted.
However, although the aforementioned conventional techniques may possibly melt the frozen state of condensed water in the fuel cell stack, it may not be possible to cope with the frozen states of the check valve provided in the gas passage of the exhaust gas from the fuel cell or the solenoid of the exhaust valve (a purge valve). When the outside temperature is below zero, the water contained in the exhaust gas is frozen, and the control valves will fail to function due to immobilization by frozen water.
It is therefore an object of the present invention is to solve the aforementioned problems and to provide a start control device for fuel cells capable of efficiently performing a warm-up operation at the time of starting a fuel cell and capable of efficiently starting the fuel cell.
In order to overcome the above described problems, the first aspect of the present invention provides a start control device for a fuel cell system which comprises an oxidant gas supply device (for example, air supply portion 12 in the embodiment) for supplying to the fuel cell the oxidant gas (for example, air, in the embodiment) obtained by adiabatic compression, control valves provided in the gas passage for discharging a reacted gas (for example, a reacted fuel gas being discharged in the embodiment) discharged from the fuel cell, a control valve heating device (for example, step S09 in the embodiment) for heating said control valves (for example, a check valve 23 and a discharge valve 24 in the embodiment) by heat exchange with the oxidant gas supplied from the oxidant gas supply device.
By providing the start control device for the fuel cell as described above, when air heated by adiabatic compression by a compressor is supplied to the fuel cell as an oxidant gas, such heated air is directly blown on the control valves such as a check valve or a discharge valve, which are disposed in a passage for discharging a reacted gas from the fuel cell. Even when respective solenoids of the control valves and the discharge valve are frozen by residual water because the fuel cell is placed in a low temperature subzero external atmosphere, it is possible to defrost the frozen valves and to decrease the time required for starting the fuel cell.
In addition, the above start control device makes it possible to conduct an efficient heating operation by utilizing thermal energy obtained by adiabatic compression of the oxidant gas without providing a particular device for heating control valves.
According to the second aspect of the present invention, in the aforementioned start control device for a fuel cell system, the start control device further comprises an oxidant gas dividing supply device (for example, oxidant gas passage 28a and flow dividing passage 28c in the embodiment) for divisionally supplying the oxidant gas supplied from the oxidant gas supply device.
By providing the above-described structure, the heated oxidant gas is supplied to the fuel cell in addition to the control valve heating device, and the control valves and the fuel cell are heated so that it is possible to reduce the time required for heating the entire fuel cell system.
According to the third and fourth aspects of the present invention, the aforementioned start control device for a fuel cell system comprises a defrosted state determination device (for example, a control device described below) for determining whether the control valves in a frozen state are defrosted, an oxidant gas supply control device (for example, warm up flow dividing device 26 in the embodiment) for supplying or for stopping the supply of the oxidant gas from the oxidant gas supply device to the control valve heating device, wherein the oxidant gas supply control device stops supplying the oxidant gas from the oxidant gas supply device to the control valve heating device when it is determined by the defrosted state determination device that the control valves are defrosted.
By providing the start control device as described above, since the oxidant gas is supplied to the control valve heating device after the control valves are defrosted, it is possible to conduct a defrosting operation in an efficient manner by preventing the supply of excess oxidant gas to the control valve heating device.
According to the fifth aspect of the present invention, in the start control device for a fuel cell system, the defrosted state determination device determines the defrosted state of the control valves by change of pressure of the reaction gas detected in response to control commands to open or to close the control valves.
By providing the start control device for the fuel cell as described above, it can be determined that the control valves are defrosted by determining whether the control valves can be opened or closed in response to the opening and closing commands based on detecting the pressure at supply and discharge ports of those valves.
According to the sixth aspect of the present invention, in the start control device for a fuel cell system, the start control device further comprises a power generation start device (for example, step S08 in the embodiment) for starting power generation of the fuel cell, and the generation starting device starts power generation when it is determined by the defrosted state determination device that the control valve is defrosted.
By providing the start control device for the fuel cell as described above, since the power generation is started after confining that the control valve for controlling the discharge gas can be operated normally, the fuel cell can generate power without reducing power generation efficiency.
According to the seventh aspect of the present invention, in the start control device for a fuel cell system, a plurality of control valves are integrally arranged in a common box (for example, a warm-up box in the embodiment), in which the oxidant gas supplied from the oxidant gas supply device can be distributed.
By providing the start control device for the fuel cell as described above, since plural control valves are integrally arranged in a common box, the heated oxidant gas can be efficiently used for heating these control valves, eliminating diffusion of the heated oxidant gas.